Battery Fuel Gauge Apparatus

ABSTRACT

A battery fuel gauge apparatus comprises a current amplifier formed by a first transistor and a second transistor. Both transistors operate in the same operation conditions except that the second transistor has a smaller channel width in comparison with that of the first transistor. The first transistor is connected in series with a battery pack. The second transistor is connected in series with a sensing device. The sensing device comprises a first resistor and a second resistor connected in series. The first resistor has a positive temperature coefficient and the second resistor has a negative temperature coefficient.

BACKGROUND

A variety of battery powered portable devices, such as mobile phones,notebook computers and the like, have become popular. Each portabledevice may employ a plurality of rechargeable battery cells. Theplurality of rechargeable battery cells may be connected in series or inparallel so as to form a rechargeable battery pack for storingelectrical energy. Rechargeable batteries include a variety of types,such as nickel-cadmium (NiCd) batteries, nickel-metal hydride (NiMH)batteries, lithium-ion batteries, lithium-ion polymer batteries,lithium-air batteries, lithium iron phosphate batteries and the like.

Different types of rechargeable battery packs may employ differentcharging methods to charge from a depleted state to a full charged stateusing a power source such as an ac/dc adapter or a universal serial bus(USB) port. In order to have a reliable rechargeable battery pack and along cycle life, the rechargeable battery pack should operate within asafe operation region to which the rechargeable battery pack isspecified. Monitoring the remaining capacity of a rechargeable batterypack is an effective way to keep the rechargeable battery pack operatingwithin the safe operation region. More particularly, an accurateestimate of the remaining capacity of the rechargeable battery pack isimportant to battery pack users to know the amount of energy left in thebattery pack and how much more time the battery powered portable devicecan be used before the battery pack needs recharging. This is commonlyreferred to as a battery pack's State of Charge (SOC).

The capacity of a rechargeable battery pack can be calculated based uponthe electrical charge flowing into the rechargeable battery pack and theelectrical current flowing out of the rechargeable battery pack. Theelectrical charge may be monitored by a battery fuel gauge apparatus. Inaccordance with the operation principle of battery fuel gauges, batteryfuel gauges may be further divided into three categories, namely acurrent integration based fuel gauge, a voltage measurement based fuelgauge and an internal impedance measurement based fuel gauge.

A current integration based fuel gauge is based upon an integral ofcharge and discharge currents of a rechargeable battery pack. Moreparticularly, the battery fuel gauge apparatus detects the electricalcharge by means of a sensing device such as a shunt resistor, a HallEffect transducer, a giant magnetoresistance (GMR) sensor and the like.Furthermore, an analog-to-digital converter (ADC) may convert the analogsignal detected by the battery fuel gauge into a digital signal and feedthe digital signal to a microprocessor in which a variety ofrechargeable battery fuel gauge algorithms may be employed to calculatethe state of charge (SOC) of the rechargeable battery pack.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a rechargeable battery fuel gaugesystem in accordance with an embodiment;

FIG. 2 illustrates a schematic diagram of the charge phase circuit 102Aof the battery fuel gauge apparatus 102 shown in FIG. 1;

FIG. 3 illustrates a schematic diagram of the battery phase circuit 102Bof the battery fuel gauge apparatus 102 shown in FIG. 1;

FIG. 4 illustrates a block diagram of the detection circuit 104 shown inFIG. 2;

FIG. 5 illustrates a schematic diagram of the sensing device 204 inaccordance with an embodiment; and

FIG. 6 illustrates in detail a schematic diagram of the current mirror202 and the protection device 110 shown in FIG. 2.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, a battery fuel gauge apparatus. Theinvention may also be applied, however, to a variety of rechargeablebattery packs.

Referring initially to FIG. 1, a block diagram of a rechargeable batteryfuel gauge system is illustrated in accordance with an embodiment. Therechargeable battery fuel gauge system 100 may comprise a rechargeablebattery 112, a protection device 110 and a battery fuel gauge apparatus102. The rechargeable battery 112 is coupled between an output terminalof the protection device 110 and ground. The rechargeable battery 112may be a nickel-cadmium (NiCd) battery, a nickel-metal hydride (NiMH)battery, a lithium-ion battery, a lithium-ion polymer battery, alithium-air battery, a lithium iron phosphate battery and the like. Itshould be noted while FIG. 1 shows the rechargeable battery 112 is asingle cell, the rechargeable battery 112 may comprise a plurality ofrechargeable battery cells connected either in series or in parallel.

The protection device 110 is placed between a terminal of the batterypack 112 and a battery charger (not shown). The protection device 110may comprise a pair of back-to-back connected p-type metal oxidesemiconductor (PMOS) transistors. When the pair of back-to-backconnected PMOS transistors is turned off, the pair of back-to-backconnected PMOS transistors can block conduction of current in eitherdirection so as to isolate the battery pack 112 from external circuits(not shown). On the other hand, when the pair of back-to-back connectedPMOS transistors is activated, the pair of back-to-back connected PMOStransistors can provide a conductive channel for charge and dischargecurrents. It should be noted that the current flowing through theprotection device 110 is equal to that of the battery pack 112. As aresult, the discharge and charge of the battery pack 112 can be measuredby monitoring the current flowing through the protection device 110.

It should be noted that while the exemplary embodiment of the protectiondevice 110 is a pair of back-to-back connected PMOS transistors, thepresent invention is applicable to protection devices formed by othersystem configurations such as a pair of back-to-back connected NMOStransistors and the like. It should further be noted that the presentinvention is also applicable to a variety of derivatives of theprotection configuration described above. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.For example, the scope of the present invention may extend to aprotection device coupled between the negative terminal of the batterypack 112 and ground.

The battery fuel gauge apparatus 102 is coupled to the protection device110. As shown in FIG. 1, the battery fuel gauge apparatus 102 furthercomprises a charge phase circuit 102A and a battery phase circuit 102B.Both the charge phase circuit 102A and the battery phase circuit 102Bare coupled to a detection circuit 104 through a plurality of switches,namely SW1, SW2, SW3 and SW4. When the battery operates in a chargemode, switches SW1 and SW2 are turned on and switches SW3 and SW4 areturned off, as a result, the detected signal from the charge phasecircuit 102A is sent to the detection circuit 104. On the other hand,when the battery operates in a battery mode, switches SW1 and SW2 areturned off and switches SW3 and SW4 are turned on, as a result, thedetected signal from the battery phase circuit 102B is sent to thedetection circuit 104.

In accordance with an embodiment, the fuel gauge apparatus 102 mayreplicate the current flowing through one PMOS transistor of theprotection device 110 using a current mirror. The current minor (notshown) may comprise a transistor with a ratio of m to M in comparisonwith the PMOS transistor of the protection device 110. By scaling downthe current flowing through the charger 110 by a ratio of M to m, thepower consumption of the fuel gauge apparatus 102 is reducedaccordingly. The detailed operation of the fuel gauge apparatus 102 willbe described below with respect to FIG. 2 and FIG. 3.

The rechargeable battery fuel gauge system 100 may further comprise adetection circuit 104, a coulomb counter 106 and a microprocessor 108.The detection circuit 104, the coulomb counter 106 and themicroprocessor 108 are connected in cascade to form a processing unit.The input terminal of the processing unit is coupled to the battery fuelgauge apparatus 102. Through the detection circuit 104, an analog signalgenerated by the battery fuel gauge apparatus 102 may be amplified to anappropriate level. Furthermore, the amplified analog signal may beconverted into its corresponding digital signal.

A coulomb counter 106 is connected in series with the detection circuit104. The coulomb counter 106 may be a highly accurate sigma-deltaanalog-to-digital converter (ADC), which is used to measure charge anddischarge currents of the battery pack 112. As known in the art, thecoulomb counter 106 is capable of providing two modes of operation. Whenthe coulomb counter 106 operates in an instantaneous current conversionmode, the coulomb counter 106 can provide the value of the instantaneouscurrent flowing through a battery as well as the voltage across thebattery. In contrast, when the coulomb counter 106 operates in anaccumulated current conversion mode, the coulomb counter 106 can providean average value of the current flowing through the battery during aselected period. For example, the selected period can be 256, 512 or1024 milliseconds.

The microprocessor 108 has an input coupled to the output of the coulombcounter 106. As known in the art, a variety of battery fuel gauge powermanagement algorithms may be embedded in the microprocessor 108. Byemploying the fuel gauge power management algorithms, the microprocessor108 may calculate the remaining capacity of the battery pack 112 so asto report the state of charge (SOC) of the battery pack 112.Furthermore, the microprocessor can take into consideration theenvironmental and device specific information of a battery to calculatethe remaining capacity for each type of battery.

FIG. 2 illustrates a schematic diagram of the charge phase circuit 102Aof the battery fuel gauge apparatus 102 shown in FIG. 1. The chargephase circuit 102A may comprise a current minor 202, a sensing device204, an n-type metal oxide semiconductor (NMOS) transistor N1 and anoperational amplifier 206. The current mirror 202 is coupled to theprotection device 110. The current mirror 202 is used to scale down thecurrent flowing through the protection device 110 to a lower level sothat the power loss at the sensing device 204 can be reducedaccordingly. The charge phase circuit 102A further comprises a firstswitch SW1 and a second switch SW2. As shown in FIG. 2, when therechargeable battery 112 operates in a charge mode, both the firstswitch SW1 and the second switch SW2 are enabled. As shown in FIG. 2,the enabled switches SW1 and SW2 allow the detection circuit 104 toreceive a signal detected across the sensing device 204. On the otherhand, the battery phase circuit 102B is inactivated by turning off botha third switch SW3 and a fourth switch SW4. The detailed operation ofthe battery phase circuit will be discussed below with respect to FIG.3.

The battery fuel gauge apparatus 102 further comprises a fifth switchSW5 and a sixth switch SW6. As shown in FIG. 2, the fifth switch SW5 iscoupled between a first terminal of the current minor 202 and a firstterminal of the protection device 110. Similarly, the sixth switch SW6is coupled between a second terminal of the current minor 202 and asecond terminal of the protection device 110. Control signals CHG andCHG are employed to control the operation of the fifth switch SW5 andthe sixth switch SW6. More particularly, CHG is the inverse of CHG. Asshown in FIG. 2, when the rechargeable battery 112 operates in a chargemode, CHG turns on the fifth switch SW5 and CHG turns off the sixthswitch SW6. On the other hand, when the rechargeable battery 112operates in a battery mode, the fifth switch SW5 will be turned off andthe sixth switch SW6 will be turned on. The operation of the batterymode will be discussed in detail with respect to FIG. 3.

Referring to FIG. 2 again, in order to ensure an accurate scale-downfrom the protection device 110, the input of the current mirror 202 iscoupled to the input of the protection device 110 through a turned-onswitch SW5 and the output of the current mirror 202 is forced to beequal to the output of the protection device 110 by employing theoperational amplifier 206. More particularly, when uneven voltages attwo inputs of the operational amplifier 206 occur, the operationalamplifier 206 may adjust the voltage across the NMOS transistor N1 byadjusting the voltage at the gate of the NMOS transistor N1. As aresult, the voltage across the current minor 202 is forced to be equalto that of the protection device 110. The detailed operation principlesof the battery fuel gauge apparatus 102 will be further described belowwith respect to FIG. 6.

FIG. 3 illustrates a schematic diagram of the battery phase circuit 102Bof the battery fuel gauge apparatus 102 shown in FIG. 1. A person ofordinary skill in the art will recognize that the configuration of thebattery phase circuit 102B is similar to that of the charge phasecircuit 102A except that the battery side of the current mirror 302 andthe battery side of the protection device 110 are coupled togetherthrough a turned-on switch SW6. It should be noted that while FIG. 3shows the current minor 302, the batter phase circuit 102B and thecharge phase circuit 102A may share the same current minor (e.g.,current mirror 202). Furthermore, the switch SW5 is turned off when therechargeable battery 112 operates in a battery mode. The operationprinciples of the battery phase circuit 102B is similar to the operationprinciples of the charge phase circuit 102, and hence are not discussedin further detail. It should be noted that the first switch SW1 and thesecond switch SW2 are turned off when the rechargeable battery 112operates in a battery mode. As a result, the charge phase circuit 102Ais disabled during the battery mode. It should further be noted that thebattery phase circuit 102B employs a different sensing device 304 incomparison with the charge phase circuit 102A shown in FIG. 2.Furthermore, the battery phase circuit 102B employs a p-type metal oxidesemiconductor (PMOS) transistor P3 to achieve the same function of theNMOS transistor N1 shown in FIG. 2. It should also be noted that thePMOS transistor P3 acts to balance the two inputs of the operationalamplifier 306 through the sensing device 304. In comparison with thesystem configuration of FIG. 2, the positive and negative inputs of theoperational amplifier 306 are swapped to satisfy the loop feedbackrequirements of the PMOS transistor.

FIG. 4 illustrates a block diagram of the detection circuit 104 shown inFIG. 2. In accordance with an embodiment, the detection circuit 104 maycomprise a sense amplifier 302, an ADC 304 and a digital filter 306. Asshown in FIG. 3, the sense amplifier 302, the ADC 304 and the digitalfilter 306 are connected in cascade. The sense amplifier 302 is used toamplify the signal across the sensing device 204 to a level appropriatefor the ADC 304 to convert the analog signal to its correspondingdigital signal. The digital filter 306 may be used to digitallymanipulate the output signal from the ADC 304 (e.g., adding additionalgain or altering the frequency components of the signal from the ADC304). A down-sampler (not shown) may be included in the digital filter306 to reduce the transmission rate and increase the data size of thesignal chain while the Shannon-Nyquist sampling theorem criterion isstill maintained. Additionally, the digital filter 306 provides ananti-aliasing filter in which unwanted noise may be eliminated andwanted signals may be amplified.

FIG. 5 illustrates a schematic diagram of the sensing device 204 inaccordance with an embodiment. The sensing device 204 may comprise afirst sensing resistor 404 and a second sensing resistor 406 connectedin series. The first sensing resistor 404 may be a silicided polyresistor. The second sensing resistor 406 may be a non-silicided polyresistor. In accordance with the characteristics of both resistors, thefirst sensing resistor 404 formed by silicided poly has a positivetemperature coefficient. In contrast, the second sensing resistor 406formed by non-silicided poly has a negative temperature coefficient. Byconnecting the first sensing resistor 404 and the second sensingresistor 406 in series, the resistance of the sensing device 204 mayremain approximately constant under temperature variations. One skilledin the art will understand the temperature variation compensation schemebased upon the characteristics of silicided poly resistors andnon-silicided poly resistors. Therefore, the formation of the sensingdevice 204 is not discussed in further detail herein. It should berecognized that while FIG. 5 illustrates the sensing device 204 with tworesistors connected in series, the sensing device 204 could accommodateany number of resistors connected in series, in parallel or anycombinations thereof.

FIG. 6 illustrates in detail a schematic diagram of the current mirror202 and the protection device 110 shown in FIG. 2. In accordance with anembodiment, the protection device 110 may be a PMOS transistor P1 havinga channel width equal to M. It should be noted while FIG. 6 illustratesa single PMOS transistor P1, the protection device 110 may comprise apair of PMOS transistors back-to-back connected in series. Either PMOStransistor of the back-to-back connected PMOS transistors can be used toform a current amplifier with the current minor 202. In accordance withan embodiment, the channel width of the PMOS transistor P1 is large tosupport a smaller on-resistance of the protection device 110.

In order to reduce power consumption, a PMOS transistor P2 having achannel width equal to m is paired with the PMOS transistor P1 to form acurrent minor. The gates of the PMOS transistors P1 and P2 are connectedeach other. The sources of the PMOS transistors P1 and P2 are connectedto the same voltage potential INPUT. The drains of the PMOS transistorsP1 and P2 are connected to the negative and positive inputs of theoperational amplifier 206 respectively. As a result, the operationalamplifier 206 can force the voltage at the drain of the PMOS transistorP1 equal to that of the PMOS transistor P2. Because the drains, sourcesand gates of two PMOS transistors P1 and P2 are of the same voltagepotentials, the current flowing through each PMOS transistor isproportional to its channel width. As a result, the transfer ratio ofthe current mirror formed by the PMOS transistors P1 and P2 is M to m.It should be noted that m is much smaller than M. As such, the currentflowing through the PMOS transistor P2 is much less than that flowingthrough the PMOS transistor P1.

An advantageous feature of having a current mirror with a ratio of M tom is that the power consumption at the sensing device 204 is reduced bya factor of M/m. The total efficiency of the fuel gauge apparatus can beimproved as a result. Another advantageous feature of having the currentmirror 202 is that the PMOS transistor P1 is usually already in abattery system. By employing the current minor 202, the current flowingthrough the PMOS transistor P1 can be detected without the need of anexternal resistor. As such, such a battery fuel gauge mechanism can saveon component and packaging costs.

Although embodiments of the present invention and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. An apparatus comprising: a first transistor connected in series witha battery pack; a second transistor comprising: a second gate coupled toa first gate of the first transistor; and a second output terminalcoupled to a first output terminal of the first transistor; a sensingdevice connected in series with the second transistor; a thirdtransistor connected in series with the sensing device; and anoperational amplifier comprising: a first input coupled to the firsttransistor; a second input coupled to the second transistor; and anoutput coupled to the third transistor.
 2. The apparatus of claim 1,wherein the third transistor is an n-type metal oxide semiconductortransistor comprising: a drain coupled to the sensing device; a sourcecoupled to ground; and a gate coupled to the output of the operationalamplifier.
 3. The apparatus of claim 1, wherein the third transistor isa p-type metal oxide semiconductor transistor comprising: a sourcecoupled to the sensing device; a drain coupled to the first transistor;and a gate coupled to the output of the operational amplifier.
 4. Theapparatus of claim 1, wherein the sensing device comprises a combinationof a first resistor and a second resistor.
 5. The apparatus of claim 4,wherein the first resistor has a positive temperature coefficient andthe second resistor has a negative temperature coefficient.
 6. Theapparatus of claim 1, wherein the first transistor with a first channelwidth and the second transistor with a second channel width form acurrent minor with a current ratio equal to a ratio between the firstchannel width and the second channel width.
 7. The apparatus of claim 1,further comprising: a first switch between a drain of the firsttransistor and a drain of the second transistor; and a second switchbetween a source of the first transistor and a source of the secondtransistor.
 8. A system comprising: a battery pack connected in serieswith a protection device comprising a first transistor; a battery fuelgauge apparatus comprising: a second transistor comprising: a secondgate coupled to a first gate of the first transistor; and a secondoutput terminal coupled to a first output terminal of the firsttransistor; a sensing device connected in series with the secondtransistor; a third transistor connected in series with the sensingdevice; and an operational amplifier comprising: a first input coupledto the first transistor; a second input coupled to the secondtransistor; and an output coupled to the third transistor; a detectioncircuit coupled to the sensing device; and a processing circuit coupledto the detection circuit.
 9. The system of claim 8, wherein thedetection circuit comprises: a sense amplifier having two inputs coupledto the sensing device; an analog-to-digital converter coupled to anoutput of the sense amplifier; and a digital filter coupled to an outputof the analog-to-digital converter.
 10. The system of claim 8, whereinthe processing circuit comprises: a coulomb counter having an inputcoupled to the detection circuit; and a microprocessor having an inputcoupled to the coulomb counter.
 11. The system of claim 10, wherein themicroprocessor is configured to calculate and report a capacity level ofthe battery pack.
 12. The system of claim 8, wherein the battery packcomprises a plurality of battery cells.
 13. The system of claim 8,further comprising a plurality of switches coupled between the sensingdevice and the detection circuit.
 14. The system of claim 8, wherein thefirst transistor and the second transistor form a current mirrorconfigured such that: a current flowing into the second transistor isreduced by a factor of M/m in comparison with a current flowing into thefirst transistor.
 15. A method comprising: connecting a first transistorin series with a battery pack; configuring a second transistor suchthat: the second transistor has a same operating condition as that ofthe first transistor; and connecting a sensing device in series with thesecond transistor.
 16. The method of claim 15, further comprising:connecting a third transistor in series with the sensing device;connecting an output of an operational amplifier with a gate of thethird transistor; and adjusting the gate of the third transistor when avoltage across the second transistor is not equal to a voltage acrossthe first transistor.
 17. The method of claim 15, further comprising:amplifying an analog signal across the sensing device; converting theanalog signal into a digital signal; and filtering off unwanted noise.18. The method of claim 15, further comprising: providing aninstantaneous value of a current flowing through the first transistor;and providing an average value of the current flowing through the firsttransistor.
 19. The method of claim 15, further comprising: calculatinga capacity level of the battery pack; and reporting the capacity levelof the battery pack.
 20. The method of claim 15, further comprising:forming the sensing device by a combination of a first resistor with asecond resistor, wherein the first resistor has a positive temperaturecoefficient and the second resistor has a negative temperaturecoefficient.